1. Field of the Invention
This invention relates to a control circuit for generating serial video output signals which are employed to generate a display image. More particularly, this invention relates to timing and control circuits for a random access memory (RAM) employed for multiple purposes including a user identifiable character generator, a refresh memory and display attributes.
2. Description of the Prior Art
Prior art character generators include generators that produce output signals employed to control the beam intensity of a cathode ray tube (CRT). Such signals may also be employed to control other forms of display panels. Calligraphic or strobe type generators are known. Raster scan character generators are known. Raster scan character generators are commercially available on a single integrated circuit semiconductor chip. Most such I.C. chips are preprogrammed read only memories (ROMS) which produce predetermined group output signals in response to character address input signals. Such ROM chips usually are designed to conform to ASCII font and character standards and are not capable of being changed or programmed by the user. Intelligent video display terminals (VDT) are known which are capable of being operated as a general purpose computer. Such VDT's include the operation and control of peripheral equipment such as tape drives, disk drives, printers etc. The general purpose computer in an intelligent VDT is also capable of being operated in an office information system environment. Such office information system terminals are usually capable of having access to the computer stored information.
The general purpose computers employed in intelligent video display terminals are preferably very fast and capable of being operated on a relatively high level language and operating system to achieve greater throughput than the ordinary microprocessor. As a result of these and other requirements of the intelligent video display terminal, such terminals often employ large high density and cost efficient RAM memories.
It would be desirable to utilize a portion of the large cost efficient RAM memories in an intelligent video display terminal to provide the control circuit information for the generation of video display output signals that were formerly produced by preprogrammed and dedicated ROM character generators.